The present invention is directed to a method and apparatus for protecting photosensitive material against damage from high light levels, as may be caused by lasers.
Photosensitive material is vulnerable to damage from high light levels. In some instances, the high light levels, particularly from lasers, may be intentionally produced by an adversary to damage equipment which utilizes photosensitive material.
There are many uses for photosensitive material, which for example include a photocathode for an image intensifier which may be used in a night vision device, an array of photodetectors in a video camera or other imaging device, and film in a photographic camera. While such use and others for photosensitive material are encompassed by the present invention, it finds particular application for the protection of a photocathode in an image intensifier device.
Image intensifiers are used for amplifying low light images and are employed, for example, in night vision goggles, security cameras, and medical instruments.
An image intensifier tube is typically comprised of a photocathode which is attached to a glass input faceplate or window, an electron amplifier, which is typically a microchannel plate, and a converter for converting amplified electrons to light, which is typically a phosphor screen. In the operation of the device, light including infrared (IR) is fed through the window to the photocathode. The photocathode converts the light including IR to electrons, and the electrons are amplified by the microchannel plate. Finally, the amplified electrons are incident on the phosphor screen where they create a visible image.
Image intensifiers can be very sensitive to laser damage, which arises from the laser energy being focused on the photocathode or microchannel plate. A prior art approach to protection of image intensifiers was to use a faceplate comprised of a fiber optic bundle. That is, if a solid glass faceplate is used, the objective lens focuses a small spot on the faceplate having concentrated energy which can damage the photocathode. However, when fiber optics are used in place of solid glass, the energy is distributed over a larger spot on the photocathode, and is not as damaging. This is because the small spot which is focused by the objective on the input end of the fiber faceplate is somewhat larger than the diameter of one fiber, and typically covers parts of several fibers. Even if the spot covers only a small portion of a fiber at the input end, it is emitted from the entire diameter of the fiber at the output end, so the light spot is enlarged over the photocathode.
Even though fiber optic faceplates have found use in protecting photocathodes, there are performance, integration and manufacturing disadvantages associated with their use. As to performance, the resolution and modulation transfer function (MTF) of a fiber optic tube is less than a glass tube. This is because the fibers are discrete sampling devices, and inherently have lower resolution than non-sampling devices. When several sampling devices are overlapped together, the resolution drops.
The integration disadvantages stem from the fact that the customer""s desire is to have a tube which is laser hardened, but which is form fit and functionally identical to the tubes presently fielded in night vision equipment. However, such equipment was designed around glass faceplate tubes. Thus, fiber optic tubes require that the equipment housing be replaced to allow for the longer optical length of a fiber optic tube, and the objective lenses need to be replaced, as these were designed to function properly with a glass faceplate. When the glass faceplate is replaced with a fiber optic faceplate the lens has poorer MTF and thereby the device resolution and contrast are reduced. Thus, a fiber optic faceplate tube is not form fit and function for most night vision device applications.
The manufacturing problems are due to the glass characteristics. Fiber optics are usually made of glasses of two different coefficient of expansion. Due to this, a gallium arsenide photocathode layer can give a visual appearance in the tube as though a bad paint brush had been used to deposit the layer, giving streaks, which in common parlance are called xe2x80x9cbrush linesxe2x80x9d.
It is therefore an object of the invention to provide an improved faceplate for photosensitive material, as well improved devices which incorporate such faceplate.
It is a further object of the invention to provide an improved method for protecting photosensitive material.
It is still a further object of the invention to provide an improved method of manufacturing.
In accordance with an aspect of the invention, a pseudo fiber optic faceplate is provided for protecting photosensitive material from high light levels which comprises an optical structure which optically acts like a solid window at normal light levels, but which at least one of attenuates or diffuses the light incident on the photocathode as a function of inputted light amplitude.
Thus, operational and integration advantages of a fiber faceplate can be achieved, while protection of the photosensitive material is afforded.
Other aspects of the invention will become evident by referring to the following description and claims.